1. Technical Field
This disclosure relates to an air duct and an air flow system.
2. Related Art
An air duct is formed from a synthetic resin or the like, and the air passes through the inside thereof. The air duct is used as a part of a stretch of duct system or air flow system, such as an intake system of an internal combustion engine for a car, an air-conditioning system or a cooling air delivery system. In such a duct system, a duct having a duct wall formed from a non-air-permeable material is typically used. Therefore, noise generated from a noise source such as an engine, a fan or a motor propagates inside the duct. Air column resonance occurs in the duct system. Thus, there has been a demand for reducing noise.
A technique for reducing noise propagating through a duct system, which has been developed or put into practical use, includes the provision of a large-diameter chamber section or the provision of a resonance-type silencer such as a Helmholtz resonator. Further, a technique so-called “porous duct” has also been developed in the art as a technique for giving air permeability to a portion of the duct wall. According to this technique, an air-permeable portion is provided in the duct wall formed from a non-air-permeable material. A porous duct is an attempt to reduce noise propagating through the duct by preventing the air column resonance in the duct system. Further, a technique of making a hole in the duct wall so as to prevent air column resonance (i.e., a tuning hole) has been also known.
As a porous duct, a technique described in JP-A-2001-323853, for example, is known in the art. A characteristic feature of this technique lies in a porous material, such as a non-woven fabric, having a moderate air-permeability attached to a duct wall so as to cover a hole provided in the middle section of the non-air-permeable duct wall. Thus, the space inside the duct and the outside space are brought into communication with each other through the porous material. Moreover, a porous duct described in JP-A-2001-323853 includes a non-woven fabric that is heat-welded to an opening at the end of a small tubular portion that is provided so as to project from the wall surface of the duct body. With such a duct, it is possible to suppress the occurrence of air column resonance in the duct system by adjusting the air permeability of the porous material. Thus, it is possible to reduce noise propagating through the duct system. This also provides an advantage that a non-woven fabric can be easily attached, and an advantage that the air flow resistance of the duct can be reduced.
According to a technique disclosed in JP-A-2009-281166, a synthetic resin duct having a duct wall formed from a thermoplastic resin is firstly molded. Then, at least a portion of the duct wall is processed as a laser light-irradiated portion. That is, a small aperture section, which includes a plurality of small apertures formed to be lined in an array by a laser drilling process, is formed in the irradiated portion. According to this technique, the air column resonance of the duct can be prevented, and thus the noise propagating through the duct can be reduced.